Fueled by emerging bandwidth-hungry applications and increases in computer processing power, internet traffic has sustained exponential growth in recent years. Different multiplexing techniques such as time-, wavelength-, and polarization-division multiplexing have been investigated in detail, both theoretically and experimentally, to manage this growth. Another technique for a higher link capacity that has been considered is increasing the fiber count in a fiber cable. Recently, space-multiplexed optical transmission, for example using multimode and multi-core fibers, has attracted great interest due to its ability to multiply fiber capacity. Multimode fibers comprise a single large area core that enables many spatial modes to travel inside, and multi-core fibers comprise several cores. Unfortunately, space-multiplexed optical transmission remains limited to several tens of kilometers due to the lack of a practical amplification technique.
It is clear that commercial erbium-doped fiber amplifiers (EDFAs) cannot be used in space-multiplexed transmission. The basic reason for this is that commercial EDFAs are based on single-mode fibers, which have only one degree of freedom (i.e., one spatial mode) while multimode and multi-core fibers have multiple (e.g., many) degrees of freedom. A straightforward way to provide signal amplification for a multi-core fiber is to separate the cores and then amplify each core individually using separate, dedicated single-mode EDFAs. This method increases the degrees of freedom but also increases the number of components that are required by a factor equal to the number of cores (N). This means that there needs to be N times the pump sources, N times the wavelength-division multiplexing (WDM), N time the erbium-doped fibers (EDFs), and N times the electronics. It would be desirable to avoid such multiplicity of components.